The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
A homogeneous charge compression ignition (HCCI) refers to a form of internal combustion within an internal combustion engine. HCCI compressing a mixture of fuel and oxidizer to a point of auto-ignition. The auto-ignition release chemical energy that is translated into work and heat. In an HCCI engine ignition occurs at several places at a time which makes a fuel/air mixture burn nearly simultaneously. An HCCI engine performs closer to an ideal OTTO cycle, provides improved operating efficiency (operates leaner), and generates less emissions than spark ignition engines. However, since there is no direct initiator of combustion, the ignition process is inherently challenging to control.
To achieve dynamic operation in an HCCI engine a control system may alter the conditions that induce combustion. For example, a control system may adjust compression ratios, induced gas temperature, induced gas pressure, or the quantity of retained or reinducted exhaust. Several approaches have been used to perform the stated adjustments and thus extend the HCCI operating region by providing finer control over temperature-pressure-time histories within a combustion chamber.
One approach is variable valve timing. Compression ratios can be controlled by adjusting when intake valves close. The amount of exhaust gas retained in a combustion chamber can be controlled by valve re-opening and/or valve overlap. Variable valve timing is limited in control over the auto-ignition process.
Another approach that is used to further increase control is referred to as a “2-step” intake valve lift approach. The 2-step intake valve lift approach includes switching intake valve modes of operation between a HIGH lift mode and a LOW lift mode, which have corresponding lift profiles. During the HIGH lift mode, the intake valves are lifted to a HIGH level to allow for a predetermined volume of air to enter the corresponding cylinders. During the LOW lift mode, the intake valves are lifted to a LOW level, which allows a smaller predetermined volume of air to enter the corresponding cylinders relative to the HIGH lift mode. Current 2-step approaches tend to exhibit inconsistent and non-uniform lift transitions and thus inconsistent end results.